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The top quark decays before it hadronizes. Before its spin state can bechanged in a process of strong interaction, it is directly transferred to thetop quark decay products. The top quark spin can be deduced by studying angulardistributions of the decay products. The Standard Model predicts thetop/anti-top quark ($t\bar{t}$) pairs to have correlated spins. The degree issensitive to the spin and the production mechanisms of the top quark. Measuringthe spin correlation allows to test the predictions. New physics effects can bereflected in deviations from the prediction. The measurement of the spincorrelation of $t\bar{t}$ pairs, produced at the LHC with a center-of-massenergy of $\sqrt{s} = 7$ TeV and reconstructed with the ATLAS detector, ispresented. The dataset corresponds to an integrated luminosity of 4.6$\textrm{fb}^{-1}$. $t\bar{t}$ pairs are reconstructed in the $\ell$+jetschannel using a kinematic likelihood fit offering the identification of lightup- and down-type quarks from the $t \rightarrow bW \rightarrow bq\bar{q}'$decay. The spin correlation is measured via the distribution of the azimuthalangle $\Delta \phi$ between two top quark spin analyzers in the laboratoryframe. It is expressed as the degree of $t\bar{t}$ spin correlation predictedby the Standard Model, $f_{\textrm{SM}}$. The result of $f_{\textrm{SM}}= 1.12\pm 0.11\,\text{(stat.)} \pm 0.22\,\text{(syst.)}$ is consistent with theStandard Model prediction of $f_{\textrm{SM}}= 1.0$.

First Measurements of Spin Correlation Using Semi-leptonic $t\bar{t}$ Events at ATLAS